Steam cracking, also referred to as pyrolysis, is used to crack various hydrocarbon feedstocks into olefins, preferably light olefins such as ethylene, propylene, and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has a firebox and radiant coil section, among other features. The hydrocarbon feedstock typically enters the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is heated and at least partially vaporized by indirect contact with hot flue gas and by direct contact with steam.
The vaporized feedstock and steam mixture is then introduced into the radiant coil section where the pyrolysis cracking chemistry primarily takes place. The resulting products comprising olefins leave the radiant coils and are quenched to halt further pyrolysis reactions.
Conventional steam cracking systems have been effective for cracking high-quality liquid feedstocks which contain fully volatile hydrocarbons, such as gas oil and naphtha. Additionally, steam cracking economics sometimes favor cracking lower cost feedstocks containing resids such as, by way of non-limiting examples, atmospheric residue such as atmospheric pipe still bottoms, and crude oil. For feedstocks containing resids, a vapor liquid separator, for example a vapor liquid separator as described in U.S. Pat. Nos. 7,138,047; 7,090,765; 7,097,758; 7,820,035; 7,311,746; 7,220,887; 7,244,871; 7,247,765; 7,351,872; 7,297,833; 7,488,459; 7,312,371; 6,632,351; 7,578,929; and 7,235,705, which are incorporated by reference herein in their entirety, is used to remove non-volatile components that promote coking when cracked. Cracked effluent from furnaces processing these liquid feeds can also be quenched in at least a primary transfer line exchanger (TLE). For heavier liquids, e.g., heavier naphthas and all gas-oil feeds, a direct oil quench connection is often required downstream of the primary TLE. The oil quench connection allows addition of quench oil into the pyrolysis product stream to provide heat transfer from the product stream directly to the injected quench oil.
A problem with direct oil quench connections is the tendency to plug rapidly when the relatively cold quench oil contacts the hot pyrolysis effluent. Oil quench fittings have been designed as a specialized technology for the addition of oil into the furnace effluent in a manner that does not cause rapid plugging.
Steam cracking, especially steam cracking of heavier feeds, such as kerosenes and gas oils, produces large amounts of tar, which leads to rapid coking in the radiant coils, TLE, and quench connection of the furnace. Frequent feed interruptions are required to enable coke removal through a process known as decoking. Within the industry, the normal method of removing coke from the radiant coils of a cracking furnace is decoking. During this process, hydrocarbon feed is interrupted to the furnace and steam passes through the furnace. The furnace effluent is redirected from the recovery section of the olefins plant to a decoking system. Air is added to the steam passing through the furnace and the heated air/steam mixture removes the coke deposits by controlled combustion. The decoke effluent is eventually exhausted to the atmosphere, either via a decoke drum, e.g., a cyclonic separator, or via the furnace firebox and stack.
During the decoking of a furnace having a quench fitting, it is not possible to add quench oil through the quench fitting to cool the radiant coil and TLE effluent, as release of the quench oil to atmosphere is unacceptable. It is conventional to add water into the quench fitting to cool the decoking effluent. However, much less water is required to cool the decoke effluent than quench oil required to cool the pyrolysis effluent. Maldistribution results when the flow of water is below the quench fitting design oil flow rates. The maldistribution leads to stratified flow in the downstream piping where the vapor stream in the majority of the pipe cross section is much hotter than the liquid stream running along the bottom of the pipe. This leads to uneven and variable temperature gradients around the pipe which, over time, can lead to thermal fatigue failures of the pipe and flange leaks. Additionally, the large temperature difference between the vapor and liquid streams in the piping makes control of the quench water addition rates to maintain a stable temperature difficult since a temperature sensor in the pipe is contacted by the hot vapor or relatively cold liquid stream. The rapid cycling of measured temperature leads to rapid cycling of quench water addition rate, further increasing the thermal fatigue of the piping.
Therefore, a process is desired for removing coke from a steam cracking furnace having a transfer line exchanger and oil quench connection that reduces or prevents stratified flow, provides improved decoking effluent temperature control and reduces mechanical fatigue of piping downstream of the oil quench fitting.